Has anyone hired a private guide in Oslo?
I'm wondering why you think you need the services of a private guide in Oslo. We've done well self guiding with the aid guides such as the RS Scandinavia guide book and free TI information booklets and maps.
Most Norwegian (at least the native born) speak English and are friendly to tourist. The transportation system T-bane, busses and trolleys are easy to use and central Oslo is easily walked. Oslo is also pretty safe so you don't need a guide to beat off the undesirables.